October 08, 2004

Fall color extravaganza still a possibility

by K.C. Jaehnig

CARBONDALE, Ill. - - With parts of the region abnormally dry, will Southern Illinois produce its traditional fall color extravaganza? Maybe. Maybe not. Dry conditions and cool nights usually help pigment production along, but you can get too much of a good thing, notes Southern Illinois University Carbondale forester James J. Zaczek.

"In a drought like we're having here, when the leaves dry up and fall off, there's nothing there to color up even if you start getting some rain," he says.

"Trees also begin to drop their leaves if it gets too cold - - down below the freezing point."

Still, the lack of rain isn't affecting all trees equally.

"It depends on the species," Zaczek says.

"Some trees, like yellow poplar (tulip trees) and cottonwoods, are drought deciduous - - they drop their leaves as a protection mechanism so that the drought doesn't kill them. Trees that tend to grow on moist sites are the ones that are most likely to do this.

"Other species are more tolerant of dry conditions - - they just slow down and hang onto their leaves. Oaks are like that - - they have mechanisms that keep their leaves from dying out so they can function under extremely low water availability."

Hickories also can weather drought, as can some maples, depending in part on their genetic inheritance and on whether they're growing on dry or wet sites. "There still are a lot of leaves out there in relatively decent condition," Zaczek says. "That could set us up for a nice fall."

Still, whether the leaves color up or wither away, they matter less to a tree than its innards. Given the lack of rain over the last 90 days - - average rainfall for some areas of Southern Illinois is 3 inches below normal - - the rest of the tree could be in trouble. Prolonged dry spells equal tree stress, and stress - - as with humans - - makes trees more vulnerable to disease.

Younger trees and older trees are a little more at risk. The "babies" don't have the well-developed root systems that can find and suck up scarce water; the "senior citizens" are just so darn big they can't get enough water to meet all their needs.

"But age isn't everything," Zaczek says. "A young tree without much competition from other trees or grass cover can be fairly vigorous even when rain has been scarce."

You can't water a forest, Zaczek points out to people who have been calling with tree concerns, but if you have woodland species in your landscape (Zaczek himself has a black oak of which he's quite fond), you can water individual trees.

"Most forest trees have most of their fine absorbent roots in the upper soil horizon, but some roots may be deeper, so it's better to water less frequently but with more water," he says. "If you water lightly and frequently, the grass will get most of it."

You won't have to water for long though. Zaczek says the dormant season, when trees begin shutting down for the winter, is just around the corner. "Once the leaves are off the trees, they'll lose very little water," he adds.